Conventional active noise reduction (ANR) headsets utilize acoustic output generated by the headsets' electroacoustic transducers to minimize the user's perception of ambient noise. For example, a conventional ANR headset has a noise cancelling assembly associated with each electroacoustic transducer. The noise cancelling assembly can include a microphone mounted in proximity to the electroacoustic transducer within each of the headset's ear cups. During operation, the microphones receive an audio input as heard by the user and associated electronics filter the resulting audio signal, based on the principle of feedback control, to generate a noise cancelling signal. The noise cancelling assembly feeds the noise-cancelling signal to the electroacoustic transducer amplifier, which, in turn, combines the noise-cancelling signal with desired audio from an audio source, if one is present. The noise-cancelling signal creates destructive interference with ambient noise, such as noise generated external to the headset, as the ambient noise and the combined audio signal arrive at a user's ear. The noise cancelling assembly thus minimizes the ambient noise perceived by the user and allows the user to experience a substantially clear audio input from the audio source.
In use, when placed over the user's ears, the ear cups associated with the headset form a seal between the user's head and a volume containing each of the electroacoustic transducers. The air captured between the user's ears and each electroacoustic transducer acts as a spring having a relatively high spring constant such that the air reduces displacement or excursion of each electroacoustic transducer during operation (i.e., as compared to the effect of the air captured between the user's ears and the electroacoustic transducer if the ear cup were not sealed to the head). However, in certain cases, one or both of the ear cups may not be completely sealed against the user's head. In such a case, because the volume within the ear cup is exposed to external, atmospheric air pressure, the air captured between the user's ear and the electroacoustic transducer acts as a spring having a relatively low spring constant. In certain cases an associated ANR headset amplifier is designed to provide high amplitude signals to the electroacoustic transducer in order to cancel high levels of noise under normal wearing conditions, such as when the ear cups form a relatively tight seal to the user's head. During conditions where the seal is not as tight (e.g., is leaky) the resulting decreased spring constant of the air captured between the user's ear and the electroacoustic transducer can allow the transducer to be over-extended even with a normal voltage drive signal level.
To minimize clipping or distortion when using a relatively large voltage drive signal, conventional ANR headsets utilize a compressor to reduce ANR loop gain when the drive signal voltage crosses a threshold that approaches the amplifier clipping limit. Typically, for conventional ANR headsets utilized in relatively high-noise environments, the voltage threshold is based upon the maximum drive signal voltage that the electroacoustic transducer or driver can safely tolerate at low frequencies in free air or unloaded conditions. This is done to protect the electroacoustic transducer or driver from potentially damaging levels of displacement. By reducing ANR loop gain when clipping is approached, the driver is protected and clipping, and the unpleasant audio artifacts therefore, can also be prevented. Accordingly, by reducing the loop gain to the electroacoustic transducer amplifier based upon the drive signal voltage the headset minimizes both clipping of the audio signal output and potential damage to the electroacoustic transducer components as caused by over actuation.